In recent years, techniques to form thin film transistors (TFT) using a semiconductor thin film (with a thickness of approximately several nanometers to several hundreds of nanometers) which is formed over a substrate having an insulating surface have attracted attention. Thin film transistors are widely applied to electronic devices such as ICs and electro-optic devices and are particularly expected to be rapidly developed as switching elements of image display devices. Various metal oxides are used for a variety of applications. Indium oxide is a well-known material and is used as a transparent electrode material which is necessary for liquid crystal displays and the like.
Some metal oxides have semiconductor characteristics. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide. Thin film transistors in each of which a channel formation region is formed using such metal oxide having semiconductor characteristics have been known (Patent Documents 1 and 2).
Transistors to which oxide semiconductors are applied have relatively high field effect mobility among transistors to which other semiconductors having amorphous structures are applied. Therefore, driver circuits of display devices and the like can also be formed using the transistors to which oxide semiconductors are applied.